1. Field of the Invention
The present invention relates to a driving device for a vibration type actuator such as a vibration wave motor and, more particularly, to a driving device for detecting vibration state and controlling the alternating signal to be applied to a piezoelectric element serving as a driving electro-mechanical energy converting element portion.
2. Related Background Art
Recently, a vibration type actuator such as a vibration wave motor has been developed and put into practice by the present applicants. This well know vibration type actuator uses an electro-mechanical energy converting element such as a piezoelectric element or electrostrictive element to generate high-frequency vibration by applying an alternating signal thereto, and extracts the vibration energy as continuous mechanical motion. Since this operation principle is known, a description thereof will be omitted.
FIG. 9 is a side view a conventional bar-shaped vibration wave motor as a vibration type actuator and also shows a wiring diagram of an arrangement for applying a voltage to a piezoelectric element and extracting the output voltage. A vibrator 101 is a constituent of the bar-shaped vibration wave motor and is a combination of a piezoelectric element or electrostrictive element and an elastic member.
The piezoelectric element portion of the vibrator 101 is constituted by A- and B-phase driving piezoelectric elements a1, a2, b1, and b2 and a vibration detection piezoelectric element S1. This piezoelectric element is driven by respectively applying an A-phase voltage to an electrode plate A-d sandwiched between the A-phase piezoelectric elements a1 and a2 and a B-phase voltage to an electrode plate B-d sandwiched between the B-phase piezoelectric elements b1 and b2.
In this case, ground electrode plates GND-d are arranged on the rear sides of the A-phase piezoelectric elements a1 and a2 and the B-phase piezoelectric elements b1 and b2 to set the GND potential.
Similarly, one side (on the B-phase side) of the vibration detection piezoelectric element S1 is set at the GND potential, and a signal is extracted from an electrode plate S-d on the other side. The signal extraction side (on the electrode plate S-d side) of a vibration detection piezoelectric element S is in contact with a metal block. This block is insulated from the GND potential by an insulating sheet. With this structure, an output voltage corresponding to the vibrations of the vibration detection piezoelectric element S can be directly obtained therefrom. A resonant frequency or the like is then obtained from the magnitude of this voltage or the phase difference between itself and the driving voltage.
FIG. 10 shows a driving circuit for such a vibration wave motor. This circuit includes an oscillator 2 for generating an alternating voltage, a 900 phase shifter 3, switching circuits 4 and 5 for switching alternating voltages (signals) from the oscillator 2 and the 900 phase shifter 3 with a power supply voltage, and step-up coils 6 and 7 for amplifying pulse voltages switched by the switching circuits 4 and 5.
This circuit also includes a phase difference detector 8 for detecting the phase difference between signals from the A-phase driving electrode and the vibration detection piezoelectric element S and a control microcomputer 10 for instructing the oscillator 2 to supply an alternating signal having a given frequency at which the vibration wave motor is to be driven. Signals from the driving electrode A and the vibration detection electrode S are regular sine waves. These signals are converted into square waves by high-voltage comparators 11 and 12. The phase difference detector 8 can output a signal corresponding to the phase difference between these waves to the microcomputer 10. The microcomputer 10 determines any deviation from the resonant frequency at present using this signal, and performs control to drive the motor at an optimal frequency. In this manner, driving frequency control can be performed.
In addition, since the vibration wave motor uses a piezoelectric element, the driving voltage is undesirably high. As a means for solving this problem, the use of a floating structure like the one shown in FIG. 11, by which the motor can be driven at a voltage about 1/2 that required in the prior art, has been considered. This structure is known and hence will be briefly described below. Piezoelectric elements al and b1 are respectively sandwiched between electrode plates A-d and A'-d and between electrode plates B-d and B'-d while the upper and lower surfaces of the piezoelectric elements are in contact with these electrode plates. However, the adjacent electrode plates A'-d and B-d are insulated from each other by an insulating sheet.
FIG. 12 shows a driving circuit for such a vibration wave motor. This circuit includes driving electrodes A, A', B, and B' for applying alternating voltages to the piezoelectric elements or electrostriction elements, an oscillator 2 for generating an alternating voltage, a 90.degree. phase shifter 3, switching circuits 4A, 4A', 5B, and 5B' for switching alternating voltages from the oscillator 2 and the 90.degree. phase shifter 3 with a power supply voltage, and step-up coils 6 and 7 for amplifying pulse voltages switched by the switching circuits 4A, 4A', 5B, and 5B'.
This circuit also includes a control microcomputer 10 for instructing the oscillator 2 to apply an alternating voltage having a frequency at which the vibration wave motor is to be driven. In this case, signals having a phase difference of 180.degree. are input to the switching circuits 4A, 4A', 5B, and 5B' to perform switching operation at the corresponding timing. At this time, an apparent voltage twice the power supply voltage is applied to each of the driving electrodes A, A', B, and B' of the vibrator through a corresponding coil. The motor can therefore be driven at a voltage 1/2 that required in the prior art.
As a vibration state detection means in this structure, a differential comparator for obtaining phase information from the difference between the voltages across two ends of each of the driving and vibration detection piezoelectric elements is used to convert the respective waveforms into square waves, as shown in FIG. 12. By detecting the phase difference between these square waves, any deviation from the resonant frequency can be determined.